This week's activity will get you in a spin, and help you explore the centrifugal force. Take a transparent plastic beaker and lid. Put a couple of elastic bands around the top.
Take a long piece of string and attach each end to the elastic bands, on opposite sides of the beaker. The string will need to be long enough to then easily swing the beaker in a vertical circle.
Ask your students to predict what would happen if you put water in the beaker and spun it in a circle. Add water to the container until it is a quarter full and add a drop of food colouring, so the liquid can be easily seen.
Put the lid on the container and spin it in a vertical circle. Ask the children if they can see what is happening to the liquid. They may see that the liquid stays in the bottom of the container.
Now remove the lid and ask the children what will happen to the water when you spin the container now. Try it. You should find that the water remains in the container.
This behaviour is described by the centrifugal force that pulls the water away from the centre of rotation. (You are exerting a force on the string to keep the container moving in a circle. The water would continue moving in a straight line, but the container is moving in a circle and pushes against the water.)
Make a hole in the bottom of the container that's small enough to avoid water leaking out of it. What will happen when you spin the container now? You should find that water sprays out of the hole.
You may prefer to do this experiment with a much bigger container. It can be very impressive, but you may need to practise first. It's a good idea to do it outdoors, where it doesn't matter if you get wet!